A nephelometric turbidimeter determines the concentration of solid particles suspended in a liquid sample within a sample vial by projecting a measurement light beam into the liquid sample within the vial. An optical turbidity sensor which is provided outside the cuvette body detects the amount of light scattered by the suspended solid particles at an angle of, typically, 90° with respect to the light beam axis. In a process turbidimeter, the liquid sample within the sample volume defined by the sample vial is exchanged continuously, quasi-continuously, or periodically to continuously supply a control circuit with turbidity values for controlling the turbidity of the liquid. The measurement frequency or response time of the turbidimeter is defined by the sample exchange time, which is the time needed to completely exchange the sample within the vial in a defined grade as, for example, by 99%. The higher the measurement frequency of the turbidimeter is, the shorter is the reaction time of the control circuit, and the shorter the reaction time of the control circuit is, the better is the control quality of the control circuit.
In a state of the art nephelometric process turbidimeter, a separate vial head is provided on the vial opening to thereby define, together with the sample vial, a closed sample volume. The vial head is provided with a sample inlet opening and a sample outlet opening, both being located at the end wall of the vial head, the end wall lying in a cross plane with respect to the longitudinal axis of the cylindrical sample vial. The sample exchange time of this arrangement ranges from 30 to 120 seconds.